Electrochemical devices include electrochemical cells, such as fuel cells and batteries, redox batteries, flow-through capacitors, and energy storage capacitors. In order to achieve higher energy and charge densities in electrochemical devices, a need exists for greater energy and charge storage per unit volume or mass. Increased capacitance, in particular, is highly beneficial to many types of electrochemical cells, because higher capacitance increases the charge storage per unit mass or volume. Therefore, it is the object of the invention to provide electrochemical devices of very high capacitance.
The invention features an electrochemical device which includes at least two capacitor electrodes, each of which includes a conductive material characterized in that at least ten percent (10%) of the overall surface area of the conductive material is an edge plane. Any one or more of the capacitor electrodes may be an anode or a cathode. Where the device includes more than two capacitor electrodes, for example three or more capacitor electrodes, the capacitor electrodes can optionally be connected in series.
Generally, the surface area of a conductive material is made up of edge planes and basal planes. A conductive material suitable for use in a capacitor electrode of the invention is selected for a surface area in which edge plane is favored over basal plane. The edge planes are at least ten percent (10%) of the surface area of a material that is suitable for use in the invention, preferably 25%, or more preferably, 50% or higher. In contrast to a basal plane, the electric field along an edge plane is distorted so as to exhibit an xe2x80x98edge effectxe2x80x99 or xe2x80x98fringe effect.xe2x80x99
Capacitance is a property of space whereby two conductors that are separated by an insulator store charge. Charges along conductive material that contains edges, corners, or points are greatly increased as compared to other surface charges. Due to the formula C=Q/V, increased charge density is tantamount to increased capacitance. Therefore, capacitor electrodes with many edges, points, corners, or fractal surfaces exhibit greater capacitance per unit volume or mass amount of capacitor electrode material, than do materials in which the surface area of the material is predominately basal plane. Thus, it is not necessary for the conductive material in the electrode to have a high surface area in order to exhibit high capacitance. The conductive material may be of either high surface area or low surface area, as long as the overall surface area has a high degree of edge plane. Whether a conductive material is characterized as having high versus low surface area is relative; low surface area carbon material is generally regarded as less than about 2000 Brunauer Emmettt Teller (B.E.T.) or for carbon black or powdered carbon generally less than about 1000 B.E.T. One advantage of using a conductive material of low surface area but high edge plane is to minimize oxidation, thereby prolonging the lifetime of the capacitor electrode.
An electrochemical device of the invention can be, for example, an electrochemical cell, e.g., a battery, a capacitor, or a flow-through capacitor.
Preferably, a capacitor electrode of the invention has a single electrode capacitance of at least 20 farads per cubic centimeter of conductive or electric material. A capacitor electrode of the invention can have a single electrode capacitance of at least 5 microfarads per square centimeter of surface area, where xe2x80x98surface areaxe2x80x99 is used to refer to the overall surface area of the entire material, edge and basal planes combined.
Conductive materials suitable for use in capacitor electrodes may be of varying shapes. The edge planes of the conductive material can be located on one or more of a branch, dendrite, fork, jagged edge, fractal edge or surface, point, spine, or protrusion in the shape of the conductive material. In one embodiment, a capacitor electrode of the invention is prepared from a conductive material that includes particles of less than about 10 microns in diameter. The particles may be of any shape; preferably, the particles are spherical or fibrous. The surface area of the conductive material can be between 20 and 3000 square meters per gram material. Surface area is preferably measured by B.E.T., iodine number, or nitrogen absorption method.
A conductive material useful in a capacitor electrode of the invention may be a form of graphite, such as pyrolytic graphite or graphite particles. Alternatively, the conductive material can be a nanofiber, e.g., a nanofiber of less than 300 nanometers in diameter, or an aligned nanofiber, or a nanotube, or a form of thin carbon fibers that are, preferably, less than 100 microns in diameter. Other useful conductive materials may be carbon black, carbon nanoparticles, nanoporous carbon, activated carbon, or forms of carbon powder. The conductive material may also be a form of graphite that is mechanically aligned.
In another embodiment, a capacitor electrode of the invention can further include a binder material. Useful binder materials can be based on polymers known to those skilled in the art. The binder material can be, without limitation, a form of latex or a phenolic resin. Alternatively, the binder material can be a perfluorocarbon, a polymer that can be fibrillated into a polymer fiber, or a polytetrafluoroethylene (PTFE) polymer, preferably in the range of 2-20%. Preferably, the binder material is formed into a flexible sheet, e.g., a sheet of less than 0.03 inches thick, for example, between 0.005 and 1 inch thick or thicker if the electrode layers are laminated or extruded together with spacer and current collector layers in order to form a capacitor monolithic composite material. The binder material can be formed into such a sheet by, e.g., extrusion, calendaring, pressing, spray coating, or by adhering the binder material onto a current collector.
In another embodiment, the edge planes in a capacitor electrode of the invention can be protected by a layer of boron or by a layer of phosphorous.
A capacitor electrode of the invention can also include a current collector, which is preferably integral with, or in the same plane as, the conductive material, whereby xe2x80x9cintegralxe2x80x9d is meant the capacitor electrode is connected to the current collector mechanically, physically, and/or electrically. A xe2x80x9ccurrent collectorxe2x80x9d or xe2x80x9ccharge collectorxe2x80x9d provides a path for an electric current to and from the active material.
In another embodiment, the conductive material is a laminate applied to a particle. Preferably, the laminate can be graphite that is electrically integral with the current collector.
In one aspect, the electrochemical device of the invention is a flow-through capacitor. The flow-through capacitor optionally further includes a spacer layer. Preferably, the flow-through capacitor has a series resistance of less than 30 ohms per square centimeter of spacer layer. The capacitor electrode of the flow-through capacitor preferably includes one or more pores through which an aqueous, conductive solution can pass through the electrode. The edge planes of the conductive material can be located on the surface of the pores. Flow-through capacitors of the invention are particularly useful for removing ions from an aqueous medium so as to purify the aqueous medium. Preferably, the flow-through capacitor is able to remove at least 90% of the ions when the medium is a solution of 0.01M NaCl, the conductive material in the capacitor electrode is carbon, and the solution is allowed to flow through the flow-through capacitor at a flow rate of at least one milliliter per minute per gram of carbon at 2 Volts.
In a related aspect, the invention features a method of removing ions from an aqueous medium by providing a flow through capacitor which uses a capacitor electrode of the invention, and allowing a source of the aqueous medium to flow through the flow-through capacitor so as to remove the ions from the medium. Preferably, such a method achieves a 90% purification rate of a 0.01M NaCl solution of over 1 ml/minute/gram carbon at 2 volts.
In another related aspect, the invention features a method of making an electrochemical device, the method including the steps of providing at least two capacitor electrodes, each of the capacitor electrodes including a conductive material characterized in that at least 10% of the surface area of the conductive material is edge plane; and positioning a source of electrical current so as to provide electrical communication between the two capacitor electrodes. Preferably, the capacitor electrode has a capacitance of at least 20 farads per cubic centimeter of electrical material. The capacitor electrode can have a single electrode capacitance of at least 5 microfarads per square centimeter of surface area. The capacitor electrode also includes a binder material. In one embodiment, the binder material is formed into a flexible sheet, e.g., a sheet of less than 0.03 inches in thickness. The binder material may be PTFE, e.g., 2-20% PTFE. The sheet can be formed by extruding, by calendaring, by pressing, by spray coating, or by adhering the sheet onto a current collector.